The ADC field includes a variety of different types of ADC data carriers and ADC readers operable to read data encoded in such data carriers. For example, data may be encoded in machine-readable symbols, such as barcode symbols, area or matrix code symbols, and/or stack code symbols. Machine-readable symbols readers may employ a scanner and/or imager to capture the data encoded in the optical pattern of such machine-readable symbols. Other types of data carriers and associated readers exist, for example magnetic stripes, optical memory tags, and touch memories.
Other types of ADC carriers include RFID tags that may store data in a wirelessly accessible memory, and may include a discrete power source (i.e., an active RFID tag), or may rely on power derived from an interrogation signal (i.e., a passive RFID tag). RFID readers typically emit a radio frequency (RF) interrogation signal that causes the RFID tag to respond with a return RF signal encoding the data stored in the memory.
Identification of an RFID device or tag generally depends on RF energy produced by a reader or interrogator arriving at the RFID tag and returning to the reader. Multiple protocols exist for use with RFID tags. These protocols may specify, among other things, particular frequency ranges, frequency channels, modulation schemes, security schemes, and data formats.
Many ADC systems that use RFID tags employ an RFID reader in communication with one or more host computing systems that act as central depositories to store and/or process and/or share data collected by the RFID reader. In many applications, wireless communications is provided between the RFID reader and the host computing system. Wireless communications allow the RFID reader to be mobile, may lower the cost associated with installation of an ADC system, and permit flexibility in reorganizing a facility, for example a warehouse.
RFID tags typically include a semiconductor device (such as a chip) having the memory, circuitry, and one or more conductive traces that form an antenna. Typically, RFID tags act as transponders, providing information stored in the memory in response to the RF interrogation signal received at the antenna from the reader or other interrogator. Some RFID tags include security measures, such as passwords and/or encryption. Many RFID tags also permit information to be written or stored in the memory via an RF signal.
When an RFID tag is operating in an interrogation (RF) field of more than one RFID reader, the RFID tag is exposed to interrogation signals that are respectively sent from the multiple RFID readers. These interrogation signals interfere with each other (e.g., “reader-on-tag” interference). For example, an RFID tag might be present both in the interrogation field of a first RFID reader sending out a first interrogation signal to read the RFID tag and in the interrogation field of a second RFID reader sending out a second interrogation signal to read some other RFID tag(s). Depending on the strength of the second interrogation signal, the second interrogation signal might interfere with the first interrogation signal, such that the RFID tag is unable to decode or otherwise properly process the first interrogation signal. This inability of the RFID tag to properly process the first interrogation signal can prevent the RFID tag from issuing an appropriate return signal in response to the first interrogation signal. As a result, the first RFID reader may be unable to read or is barely able to read the RFID tag.
This severe degradation in system performance occurs, as explained above, when one or more other interrogation signals from other RFID readers (in the vicinity of the first RFID reader) interferes with the first interrogation signal from the first RFID reader. Degradation in system performance due to interference may also occur when there are non-RFID devices (in the vicinity of the first RFID reader) that operate in the same frequency band as the first interrogation signal and that generate RF signals that propagate into the interrogation field of the first RFID reader.